Concern for the environment has led to a demand for packaging materials that are at least partially composed of postconsumer materials. As a result, polyester materials are widely recycled. However, the stream of postconsumer polyester material is of highly variable composition and level of contamination, making it difficult to reprocess such material to the high level of quality required for use in the packaging industry. Typical variations in the compositional makeup of postconsumer materials include variations in the levels of comonomers in the polyester material, variations in the color of the polyester material, variations in the level of residual catalyst substances remaining in the polyester material, and variations in the amount of contamination introduced into the polyester material prior to recycling. For example, a previous user may have stored a pesticide in a container formed from a polyester material, thereby contaminating the polyester material which is subsequently to be recycled.
Several chemical treatment techniques are known for facilitating the recycling of polyester material. Such techniques are used to depolymerize polyester material to be recycled, whereby the polyester material is reduced to monomeric and/or oligomeric components. The monomeric and/or oligomeric components may then be purified and subsequently repolymerized to produce recycled polyester material. For example, such techniques may be used to facilitate the recycling of poly(ethylene terephthalate) (hereinafter referred to as "PET"); however, it should be apparent to the routineer in the related arts that these same techniques are applicable to other polyester materials which are desirably to be recycled.
One known technique is to subject PET to methanolysis. In accordance with the methanolysis technique, PET is reacted with methanol to produce dimethyl terephthalate (DMT) and ethylene glycol (EG). The DMT and EG may be readily purified and thereafter used to produce PET containing recycled polyester material. However, most conventional commercial PET production facilities throughout the world are designed to use either terephthalic acid (TPA) or DMT, but not both, as the monomeric raw material. Thus, additional processing would be required to convert the DMT into the TPA needed as a raw material for many such facilities.
Another known technique is hydrolysis, whereby PET is reacted with water to depolymerize the PET into TPA and EG. However, it is known that certain types of contaminants generally present in recycled PET are very difficult and expensive to remove from TPA. Moreover, for those facilities designed to use DMT as a raw material, the TPA would need to be converted into DMT.
Glycolysis may also be used for depolymerizing PET. Glycolysis occurs when PET is reacted with EG, thus producing bis-(2-hydroxyethyl) terephthalate (BHET) and/or its oligomers. Glycolysis has some significant advantages over either methanolysis or hydrolysis, primarily because BHET may be used as a raw material for either a DMT-based or a TPA--based PET production process without major modification of the production facility. Another significant advantage provided by the glycolysis technique is that the removal of glycol from the depolymerization solvent is not necessary. In this connection it is to be noted that the co-produced glycol must be separated from the water in the hydrolysis process or from the methanol in the methanolysis process.
A previous shortcoming of the glycolysis technique is that BHET produced from previously used polyester can not generally be purified using conventional procedures. For example, when BHET is subjected to vacuum distillation, polymerization thereof will generally occur. Moreover, in the past, glycolysis processes for reclaiming PET depended upon strict feed specifications and/or costly processing of PET flake prior to glycolysis because the previously known processes were unable to prevent significant levels of some types of contaminants from being present in the glycolysis product. These problems are discussed by Richard et al., for example, in their article entitled Incorporating Postconsumer Recycled Poly(ethylene terephthalate) which appeared in ACS Symposium Series, 513, 196 (1992). Thus, there clearly remains a need in the art for a glycolysis process that can handle low-quality postconsumer PET.